1. Field of the Invention
The present invention relates to an image reading apparatus. More particularly, it relates to an image reading apparatus used for making a reduced copy of figures, letters, etc drawn on a white board for example. The present invention also relates to a light conductor used for such an image reading apparatus.
2. Description of the Related Art
There are various kinds of image reading apparatuses used for reading images (figures, letters, etc) printed on a paper sheet or drawn on a white board. The optical system of such an image reading apparatus may include either selfoc (self-focusing) lenses or convex lenses for focusing the given original images onto the light receiving elements incorporated in the reading apparatus. The selfoc lenses, which are designed to perform a non-inverting and non-magnifying image-reading function, are preferably used for reading out the given images with high resolution. Generally, the selfoc lenses are more expensive than convex lenses. Thus, convex lenses are preferably used when high resolution is not required.
A conventional image reading apparatus disclosed in JP-A-2(1990)-273257 is shown in FIG. 15 of the accompanying drawings. The conventional apparatus includes a plurality of light receiving elements 91 mounted on a substrate 90, a plurality of convex lenses 92, and a light source (not shown) for illuminating the linear image reading region Sa. When the image reading region Sa is illuminated by the light source, the reflected light is converged by the convex lenses 92 to focus on the light receiving elements 91. The images received by the elements 91 are reduced-size, inverted images of the original (see an original arrow Oi and the focused arrow Ri). The light receiving elements 91 output image signals whose output levels correspond to the amounts of the received light.
Thought conventional image reading apparatuses of the above type are widely used, they have been found disadvantageous in the following respect. As stated above, the conventional apparatus of FIG. 15 uses convex lenses 92 for its optical system. Thus, even if the image reading region Sa is uniformly illuminated by the light source, the image received by the light receiving elements 91 may be different in shade from the original, thereby failing to be the true image of the original. More specifically, referring to FIGS. 15 and 16, even if the original arrow Oi (see FIG. 15) is uniformly illuminated by the light source, the shade of the focused image Ri (see FIG. 16) may vary at positions. This is because light when passing through the convex lens 92 tends to be directed closer to the optical axis C of the lens 92. As a result, the central portion D1 of the arrow Ri becomes brighter than its end portions D2, D3 (FIG. 16).